two ndctl issues in emulated mode

[jchu314atgithub]

The issues are confirmed in Linux 4.17, ndctl version 60+ .

1. reconfigure create fails to create smaller size namespace, 1G or 2G;
2. "ndctl destroy" does not destroy, one could "ndctl enable" a 'destroy'ed namespace.
   Not sure how serious the issues are, after all they only exists in emulated mode, not on real hardware,
   but they cause confusions as the man page doesn't describe behavior deltas.

Details here.

ndctl list -r region0 -Nu <== start out with this 4GiB emulated region

{
"dev":"namespace0.0",
"mode":"fsdax",
"size":"4.00 GiB (4.29 GB)",
"sector_size":512,
"blockdev":"pmem0",
"numa_node":0
}

ndctl create-namespace -e namespace0.0 -f -s 1G <=== fails to reconfig to smaller size

[ 801.103894] nd_pmem namespace0.0: unable to guarantee persistence of writes
[ 801.508475] pmem0: detected capacity change from 0 to 4294967296
[ 801.827661] nd_pmem pfn0.1: unable to guarantee persistence of writes
[ 802.256473] pmem0: detected capacity change from 0 to 4225761280
{
"dev":"namespace0.0",
"mode":"fsdax",
"size":"3.94 GiB (4.23 GB)",
"uuid":"8ab76639-bfc5-4ba5-b7be-8f7755628b0a",
"raw_uuid":"00000000-0000-0000-0000-000000000000",
"sector_size":512,
"blockdev":"pmem0",
"numa_node":0
}

ndctl disable-namespace namespace0.0

disabled 1 namespace

ndctl destroy-namespace namespace0.0

destroyed 1 namespace

ndctl list -n namespace0.0 -iu <====' destroy' doesn't destroy

{
"dev":"namespace0.0",
"mode":"fsdax",
"size":"4.00 GiB (4.29 GB)",
"sector_size":512,
"state":"disabled",
"numa_node":0
}

ndctl create-namespace -f -r region0 -s 2G <=== w/o destroy, create-ns fails for lack of resource

failed to create namespace: Resource temporarily unavailable

ndctl enable-namespace namespace0.0

[ 950.702307] nd_pmem pfn0.1: unable to guarantee persistence of writes
[ 951.145507] pmem0: detected capacity change from 0 to 4225761280
enabled 1 namespace
[root@brm-x4600-03 region0]# ndctl list -n namespace0.0 -iu
{
"dev":"namespace0.0",
"mode":"fsdax",
"size":"3.94 GiB (4.23 GB)",
"uuid":"8ab76639-bfc5-4ba5-b7be-8f7755628b0a",
"raw_uuid":"00000000-0000-0000-0000-000000000000",
"sector_size":512,
"blockdev":"pmem0",
"numa_node":0
}

[stellarhopper]

@jchu314atgithub Both of these are expected, and are quirks of using a 'legacy' or label-less namespace such as one from the memmap-ss!nn option.

The size is a property of namespace labels, and so changing the size requires label support. Similarly, when destroying a normal namespace, we edit the labels to invalidate it. For an emulated namespace this is a bit of a no-op (or is equivalent to disabling the namespace).

The only thing you can do with emulated namespaces is to change its mode.

[jchu314atgithub]

That makes sense, thanks! Just a suggestion for folks unaware of the inner work:
could we add a paragraph in the ndctl man page to summarize the high level differences?

[stellarhopper]

Yes that is a good suggestion. We recently added a "Theory of operations" section in man pages for namespace operations. We can add some verbiage about label-less/legacy namespaces there.

[jchu314atgithub]

Thanks!